In the third chapter I showed that in childhood the sexes are differentiated both physically and mentally, altogether apart from the genital organs and the sexual impulse; and I pointed out that games in particular afforded indications of mental sexual differentiation. Many games, indeed, may even be regarded as direct manifestations of the sexual impulse, and I must therefore now return to the consideration of this topic; but I shall confine myself to certain phenomena observable in the animal world, since the games of animals are, in this connexion, so much simpler than those of children. Play constitutes a major part of the activities of young animals; think, for instance, of a kitten playing with a hanging tassel or with a ball, of puppies chasing one another, and of young birds playing with fluttering wings. The games of young animals often exhibit the character of love-games, and are in that case sexually differentiated. Various authors, and especially Brehm, have recorded numerous examples of this; I give here a few instances, quoted from Groos.[45] The young male, even before its testicles have developed, woos the female by movements, song, or other characteristic sounds. The female, also sexually immature, responds coquettishly to these advances of the male. Song, which Brehm regards as a sign of the awakening of love, makes its appearance at an age when the animal is still unfitted for the reproductive act. "Young magpies (_Corvus pica_) address one another in September, and often in August and in October, in consecutive clucking notes, and in this way make exactly the same kind of noise that they are always heard making in early spring just before the pairing season. The young male green woodpecker (_Picus viridicanus_) sings in September as beautifully as in April, as I have myself heard more than once; the young great spotted woodpecker (_Picus major_) may even be heard at times in autumn, just as in spring, making his characteristic tapping sound as he explores hard branches in search of insects. Both varieties of creeper begin to sing before they have changed their youthful plumage; their song closely resembles that of the adult birds in spring, but the note is somewhat shorter and weaker. Similarly, both the German varieties of crossbill commonly begin to sing before losing the plumage characteristic of youth. Young house-sparrows and hedge-sparrows not only chatter and swear at one another like the full-grown birds at pairing time, but also like the latter the young birds distend their throats, let their wings droop, peck at one another, and in fact behave as exactly as they will next spring when fully grown. Young linnets also begin to sing before losing their youthful plumage, learn to sing well during the moulting season, and often continue to warble right on into the winter; in a mild winter young linnets will sing just as well as old ones. The young woodlark begins to sing as soon as its first moulting is